C# Handles.PositionHandle example

using UnityEngine;
using System.Collections;

[@ExecuteInEditMode()]
public class LookAtPoint : MonoBehaviour {

    // LookAtPoint.cs
    // This Script has to be outside of the editor folder.
    //
    // Usage: Just Place this script on the object you want to work the handle with.
    public Vector3 lookAtPoint = Vector3.zero;

    void Update () {
        transform.LookAt (lookAtPoint);
    }
}

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using UnityEngine;
using UnityEditor;

[@CustomEditor (typeof(LookAtPoint))]
class LookAtPointEditor : Editor {
    void OnSceneGUI () {
        LookAtPoint look = (LookAtPoint) target;
        look.lookAtPoint = Handles.PositionHandle (look.lookAtPoint, Quaternion.identity);
        if (GUI.changed)
        {
            EditorUtility.SetDirty (target);
        }
    }
}